1. Field of the Invention
The present invention relates to a method for assembling a heater-equipped air-fuel ratio sensor, which is preferably installed on an exhaust gas emission system of an internal combustion engine for automotive vehicles and others.
2. Related Art
In general, to improve the exhaust gas purification efficiency, an air-fuel ratio of exhaust gas is sensed by an air-fuel ratio sensor, such as an oxygen (O2) sensor. An air-fuel ratio sensor usually comprises a cylindrical or cup-shaped sensor element having an open end and an opposed closed end with an inside chamber. A stick-like heater is inserted in the inside chamber of this sensor element.
The heater is necessary to warm up the sensor element quickly to its active temperature during an engine start-up condition where the engine is operated at a low temperature.
More specifically, a portion sensing an air-fuel ratio of exhaust gas is a particular portion of the sensor element which is subjected to the exhaust gas (i.e. gas to be sensed), normally the front end of the sensor element. Accordingly, in the detection of the air-fuel ratio, it is definitely necessary to heat up the sensitive portion of the air-fuel ratio sensor to its active temperature. Without this warmup operation, the sensor element cannot operate accurately nor its characteristics is stabilized.
For example, Japanese Patent No. 5-46498, published in 1993, discloses this kind of air-fuel ratio sensor. According to this prior art, a heater is united with a fixing metal by soldering and the fixing metal is fixed to the open end of an inside chamber by means of a coil spring.
Furthermore, Japanese Patent No. 6-3430, published in 1994, discloses another air-fuel ratio sensor. According to this prior art, a heater is fixed to an inside chamber by utilizing a fixing metal, although the heater and the fixing metal are not united.
Meanwhile, to satisfy the exhaust gas emission requirements strictly regulated year by year, it is required to detect an air-fuel ratio within a short time period even immediately after an engine is just started up. One measure for solving this requirement is to dispose a heater as close as possible to the sensitive edge of the sensor element. By doing so, heat generated from the heater is effectively used to warm up the sensitive edge of the sensor element. More specifically, it is most preferable to bring the tip end of the heater into contact with the bottom surface of the inside chamber and to keep the contact between the heater and the bottom surface.
However, there are following problems in assembling the heater in the sensor element.
There is a necessity of checking or confirming during the assembling operation that the tip end of the heater is surely brought into contact with the bottom surface of the inside chamber.
Both of the sensor element and the heater are fragile and weak against shock. Hence, if the heater is forcibly inserted into the inside chamber of the sensor element by applying an excessively large force, there will be a possibility that either the sensor element or the heater is damaged.
Accordingly, in view of above-described problems encountered in the related art, a principal object of the present invention is to provide an improved method for assembling a heater-equipped air-fuel sensor which is capable of surely bringing the tip end of a heater into contact with the bottom surface of an inside chamber of a sensor element without damaging the heater and the sensor element.
In order to accomplish this and other related objects, the present invention provides a novel and excellent assembling method for a heater-equipped air-fuel ratio sensor having various aspects as described hereinafter.
A first aspect of the present invention provides an method for assembling a heater-equipped air-fuel ratio sensor which comprises a cylindrical sensor element having an open end and an opposed closed end with an inside chamber defined therein and a stick-like heater inserted in the inside chamber of the sensor element so that a tip end of the heater is brought into contact with a bottom surface of the inside chamber, and the heater is supported to the sensor element by a metallic holder.
More specifically, the first aspect assembling method comprises the steps of inserting the heater into the inside chamber of the sensor element until the tip end of the heater is brought into contact with the bottom surface of the inside chamber, and sliding the metallic holder along an outside surface of the heater toward the bottom surface of the inside chamber until the metallic holder is engaged with the sensor element.
A function of the above-described first aspect assembling method will be explained hereinafter.
According to the first aspect assembling method, the stick-like heater is inserted into the inside chamber of the sensor element so that the tip end of the heater can be brought into contact with the bottom surface of the inside chamber. Then, the metallic holder is fixed to the sensor element.
Accordingly, it becomes possible to prevent the heater from interfering with other components of this air-fuel ratio sensor, and therefore it becomes possible to bring the tip end of the heater safely into contact with the bottom surface of the inside chamber. In other words, the heater can be smoothly installed to a predetermined position in the sensor element.
Then, the metallic holder is elastically installed on the heater under the condition where the heater keeps contact with the bottom surface of the inside chamber.
Next, the metallic holder slides along the outer surface of the heater and shifts toward the bottom surface of the sensor element. When the metallic holder is slid along the outer surface of the heater, a sliding frictional force is caused between a heater holding portion (described later) and the outer surface of the heater. With this sliding frictional force, the heater can be firmly pressed to the bottom surface of the inside chamber. The contact between the tip end of the heater and the bottom surface of the inside chamber is maintained until the metallic holder is fixed to the open end of the sensor element. Accordingly, the tip end of the heater is surely fixed while keeping contact with the bottom surface of the inside chamber.
As described above, a force acting on the bottom surface of the sensor element is substantially equivalent to a pressing force acting in the axial direction by the above-described sliding frictional force. Thus, the heater can be fixed in the inside chamber with its tip end kept in firm contact with bottom surface of the inside chamber without adding an excessive force to the sensor element. Furthermore, the sensor element and the heater can be surely prevented from being damaged during their assembly.
According to the above-described first aspect of the invention, it becomes possible to provide an assembling method for a heater-equipped air-fuel ratio sensor which is capable of surely bringing the tip end of the heater into contact with the bottom surface of the inside chamber of the sensor element, without damaging the sensor element and the heater during their assembly.
Next, it is possible to install the metallic holder to the sensor element at an intermediate portion of the wall of the inside chamber between the bottom surface and a open end (described below).
Furthermore, it is possible to fix the metallic holder to the open end region of the sensor element. In this case, the open end region the sensor element includes both of inside and outside surfaces of the sensor element as well as the opened top surface of the sensor element.
Even further, it is preferable to provide a tapered portion or a shoulder portion somewhere in the open end region or on the wall of the inside chamber of the sensor element for supporting the metallic holder.
According to preferred embodiments of the present invention, it is preferable that the above-described first aspect assembling method further comprise a step of installing the metallic holder elastically on an upper portion of the heater after the heater is inserted into the inside chamber of the sensor element.
That is, when the heater is assembled with the sensor element, the heater is first inserted into the inside chamber so that the tip end of the heater is brought into contact with the bottom surface of the inside chamber. Then, the metallic holder is elastically attached on the upper portion of the heater. Then, the metallic holder is slid along the outside surface of the heater toward the bottom surface of the sensor element. And, the metallic holder is fixed to the sensor element. With this assembling operation, the tip end of the heater can be surely brought into contact with or settled on the bottom surface of the sensor element.
Alternatively, it is preferable that the above-described first aspect assembling method further comprise a step of installing the metallic holder elastically on an upper portion of the heater in advance, before the heater is inserted into the inside chamber of the sensor element.
That is, when the heater is assembled with the sensor element, the metallic holder is elastically attached on the upper portion of the heater. Then, the heater is inserted into the inside chamber so that the tip end of the heater is brought into contact with the bottom surface of the inside chamber. Then, the metallic holder is slid along the outside surface of the heater toward the bottom surface of the sensor element. And, the metallic holder is fixed to the sensor element. With this assembling operation, the installation of metallic holder can be simplified.
Next, a second aspect of the present invention provides an assembling method for a heater-equipped air-fuel ratio sensor which comprises a cylindrical sensor element having an open end and an opposed closed end with an inside chamber defined therein and a stick-like heater inserted in the inside chamber of the sensor element so that a tip end of the heater is brought into contact with a bottom surface of the inside chamber, and the heater is supported to the sensor element by a metallic holder.
More specifically, the second aspect assembling method comprises the steps of installing the metallic holder elastically on an upper portion of the heater, inserting the heater into the inside chamber of the sensor element until the metallic holder is brought into contact with the sensor element under a condition where the tip end of the heater is not brought into contact with the bottom surface of the inside chamber, shifting the metallic holder toward the bottom surface of the inside chamber so that the heater held by the metallic holder is further inserted in the inside chamber and the tip end of the heater is brought into contact with the bottom surface of the inside chamber of the sensor element; and sliding the metallic holder along an outside surface of the heater toward the bottom surface of the inside chamber until the metallic holder is engaged with the sensor element.
In this case, the following relationship is established
xe2x80x83a less than b
where xe2x80x9caxe2x80x9d represents an axial clearance between the tip end of the heater and the bottom surface of the inside chamber at a moment the metallic holder is brought into contact with the sensor element, while xe2x80x9cbxe2x80x9d represents an axial shift distance of the metallic holder with respect to the outside surface of the heater.
According to the above-described second aspect assembling method, after the heater is inserted into the inside chamber, the metallic holder abuts the sensor element. Thereafter, the metallic holder is depressed, until the tip end of the heater is brought into contact with or settled on the bottom surface of the inside chamber. Subsequently, the metallic holder slides along the outside surface of the heater, and is soon fixed to the sensor element. Hence, the shift distance of the metallic holder is substantially the same as a later-described fixing portion of the metallic holder which is small. This is advantageous in that the overhang of the heater protruding upward from the open end can be suppressed to a shorter value and therefore the overall size of an air-fuel ratio sensor can be reduced.
Furthermore, in the same manner as the above-described first aspect assembling method, the tip end of the heater can be surely brought into contact with the bottom surface of the inside chamber of the sensor element. The sensor element and the heater can be surely prevented from being damaged during their assembly.
Moreover, as long as the relationship axe2x89xa7b is established between the above-described clearance xe2x80x9caxe2x80x9d and the shift distance xe2x80x9cbxe2x80x9d, the tip end of the sensor element is surely prevented from accidentally colliding with the bottom surface of the inside chamber.
It is preferable that the clearance xe2x80x9caxe2x80x9d is as short as possible.
Still further, it is preferable in the above-described first or second aspect assembling method that the heater is supported by a fixing jig in an upright position so that the tip end of the heater points upward, then the heater is inserted into the inside chamber of the sensor element, then the metallic holder is shifted and fixed to the sensor element, and then the fixing jig is removed from the heater.
This assembling method is suitable for an automatic assembling operation using a machine. Hence, the efficiency in the assembling operation can be improved. Especially, it is preferable that a lead wire is attached beforehand on the heater before executing the assembling operation.
In the above-described assembling method, it is possible to support the heater in an upright position so that its tip end points upward and then the sensor element is lowered from the top to assemble it with the heater. In this case, it is possible to shift the heater upward while holding the sensor element stationary.
Even further, it is preferable in the above-described first or second aspect assembling method that the metallic holder is elastically fixed to an inside surface or an outside surface of the sensor element.
In the case where the metallic holder is installed on the inside surface of the sensor element, the heater is surely prevented from being accidentally pulled out from the inside chamber, for example, by hooking the metallic holder by the jig etc.
On the other hand, in the case where the metallic holder is installed on the outside surface of the sensor element, it is surely prevented that the open end of the sensor element is closed by the metallic holder. This is advantageous in that a great amount of referential gas can be introduced into the inside chamber. Accordingly, the characteristics of the sensor element can be stabilized. Furthermore, easy introduction of the referential gas will realize a significant amount of reduction of the inside chamber volume.
Next, a third aspect of the present invention provides an assembling method for a heater-equipped air-fuel ratio sensor which comprises a cylindrical sensor element having an open end and an opposed closed end with an inside chamber defined therein and a stick-like heater inserted in the inside chamber of the sensor element so that a tip end of the heater is brought into contact with a bottom surface of the inside chamber, and the sensor element and an insulator are secured to a housing, and furthermore the heater is supported to the sensor element by a metallic holder.
More specifically, the third aspect assembling method comprises the steps of inserting the heater into the inside chamber of the sensor element until the tip end of the heater is brought into contact with the bottom surface of the inside chamber, and sliding the metallic holder along an outside surface of the heater until the metallic holder is engaged with the insulator.
In general, the sensor element is weak. The outside and inside surfaces of the sensor element are provided with electrodes and lead portions conductive with these electrodes. Both of these electrodes and lead portions are made of thin metallic films, such as Pt films, which are not strong.
In this respect, fixing the metallic holder to the insulator makes it possible to prevent the sensor element from being broken and to prevent the electrodes and lead portions from being damaged. Furthermore, as the metallic holder is installed on the rigid insulator, the fixing force of the metallic holder is enlarged and the installation is ensured.
In the same manner as the above-described first aspect assembling method, according to the above-described third aspect invention, it becomes possible to provide a method for assembling a heater-equipped air-fuel ratio sensor which is capable of surely bringing the tip end of the heater into contact with the bottom surface of the inside chamber of the sensor element, without damaging the sensor element and the heater during their assembly.
Next, it is possible to install the metallic holder to the insulator at a portion lower or more inward the upper end region thereof. In this case, the upper end region of the insulator includes both of inside and outside surfaces of the insulator as well as the top surface of the insulator.
Even further, it is preferable to provide a tapered portion or a shoulder portion somewhere in the upper end region or on the wall of the insulator for supporting the metallic holder.
According to the features of the preferred embodiments of the present invention, it is preferable that the above-described third aspect assembling method further comprise a step of installing the metallic holder elastically on an upper portion of the heater after the heater is inserted into the inside chamber of the sensor element. This is advantageous in that the assembling operation can be performed under the condition where the tip end of the heater is surely settled on the bottom surface of the inside chamber.
Alternatively, it is preferable that the above-described third aspect assembling method further comprise a step of installing the metallic holder elastically on an upper portion of the heater in advance before the heater is inserted into the inside chamber of the sensor element. This is advantageous in that the assembling operation can be simplified, and the possibility of damaging the sensor element during assembly operation can be eliminated.
Next, a fourth aspect of the present invention provides an assembling method for assembling a heater-equipped air-fuel ratio sensor which comprises a cylindrical sensor element having an open end and an opposed closed end with an inside chamber defined therein and a stick-like heater inserted in the inside chamber of the sensor element so that a tip end of the heater is brought into contact with a bottom surface of the inside chamber, and the sensor element and an insulator are secured to a housing, and furthermore the heater is supported to the sensor element by a metallic holder.
More specifically, the fourth aspect assembling method comprises the steps of installing the metallic holder elastically on an upper portion of the heater, inserting the heater into the inside chamber of the sensor element until the metallic holder is brought into contact with the insulator under a condition where the tip end of the heater is not brought into contact with the bottom surface of the inside chamber, shifting the metallic holder toward the bottom surface of the inside chamber so that the heater held by the metallic holder is further inserted in the inside chamber and the tip end of the heater is brought into contact with the bottom surface of the inside chamber of the sensor element, and sliding the metallic holder along an outside surface of the heater toward the bottom surface of the inside chamber until the metallic holder is engaged with the insulator.
In this case, the following relationship is established
a less than b
where xe2x80x9caxe2x80x9d represents an axial clearance between the tip end of the heater and the bottom surface of the inside chamber at a moment the metallic holder is brought into contact with the insulator, while xe2x80x9cbxe2x80x9d represents an axial shift distance of the metallic holder with respect to the outside surface of the heater.
According to the above-described fourth aspect assembling method, the metallic holder is fixed to the insulator. Hence, it becomes possible to prevent the sensor element from being broken and also to prevent the electrodes and lead portions from being damaged. Furthermore, as the metallic holder is installed on the rigid insulator, the fixing force of the metallic holder is enlarged and the installation is ensured.
Moreover, the shift distance of the metallic holder is the same as the later-described fixing portion of the metallic holder which is significantly small. This is advantageous in that the overall size of an air-fuel ratio sensor can be reduced.
Furthermore, in the same manner as the above-described first aspect assembling method, the tip end of the heater can be surely brought into contact with the bottom surface of the inside chamber of the sensor element. The sensor element and the heater can be surely prevented from being damaged during their assembly.
Moreover, as long as the relationship axe2x89xa7b is established between the above-described clearance xe2x80x9caxe2x80x9d and the shift distance xe2x80x9cbxe2x80x9d, it is the tip end of the sensor element is surely prevent from colliding accidentally or interfering with the bottom surface of the inside chamber.
It be preferable that the clearance xe2x80x9caxe2x80x9d is as short as possible.
Still further, it is preferable in the above-described third or fourth aspect assembling method that the heater is supported by a fixing jig in an upright position so that the tip end of the heater points upward, then the heater is inserted into the inside chamber of the sensor element, then the metallic holder is shifted and fixed to the insulator, and then the fixing jig is removed from the heater.
This assembling method is suitable for an automatic assembling operation using a machine. Hence, the efficiency in the assembling operation can be improved. Especially, it is preferable that a lead wire is attached beforehand on the heater before executing the assembling operation.
In the above-described assembling method, it is possible to support the heater in an upright position so that its tip end points upward and then the sensor element is lowered from the top to assemble it with the heater. In this case, it is possible to shift the heater upward while holding the sensor element stationary.
Even further, it is preferable in the above-described third or fourth aspect assembling method that the metallic holder be elastically fixed to an inside surface or an outside surface of the insulator.
In the case where the metallic holder is installed on the inside surface of the insulator, the heater is surely prevented from being accidentally pulled out from the inside chamber, for example, by hooking the metallic holder by the jig etc.
On the other hand, in the case where the metallic holder is installed on the outside surface of the insulator, the open end of the sensor element is a closed by the metallic holder. This is advantageous in that a great amount of referential gas can be introduced into the inside chamber. Accordingly, the characteristics of the sensor element can be stabilized. Furthermore, easy introduction of the referential gas will realize a significant amount of reduction of the inside chamber volume.
Furthermore, according to the features of the preferred embodiments of the present invention, it is preferable in the above-described first to fourth assembling methods that the metallic holder comprise a heater holding portion elastically supporting the heater, and a fixing portion elastically engaged with an inside surface of the sensor element or an inside surface of the insulator.
This arrangement is advantageous in that the metallic holder can be easily inserted into the inside chamber. Furthermore, as an elastic force is generated in the radial direction at the fixing portion, it becomes possible to fix the metallic holder surely to the sensor element. Moreover, the configurations of the heater holding portion and the fixing portion can be variously changed. For example, the fixing portion can be formed into a cylindrical or ring shape with a cutout extending along its axial direction. Such metallic holders are fabricated by bending a thin heat-resistive metal plate.
Still further, according to the features of the preferred embodiments of the present invention, it is preferable in the above-described first to fourth assembling methods that the metallic holder comprise a heater holding portion elastically supporting the heater, and a fixing portion elastically engaged with an outside surface of the sensor element or an outside surface of the insulator.
In this case, a force acting on the sensor element or the insulator is directed radially inward. Hence, this force acts as a compression force. Both of the sensor element and the insulator are made of ceramic which is strong against compression. Accordingly, it becomes possible to prevent both of the sensor element and the insulator from being damaged. As described above, the configurations of the heater holding portion and the fixing portion can be variously changed. And, the metallic holders are fabricated by bending a thin heat-resistive metal plate.
Even further, according to the features of the preferred embodiments of the present invention, it is preferable in the above-described first to fourth assembling methods that the sensor element is assembled beforehand in a housing in a subassembly condition.
With this arrangement, it becomes possible to reduce the number of assembling processes required after assembling the heater. Accordingly, it becomes possible to prevent any interference occurring in the vicinity of the portion where the heater is installed, eliminating the possibility of causing problems.
Furthermore, it is preferable to subassemble the sensor element and the insulator beforehand in the housing. With this arrangement, it becomes possible to easily and stably install the heater in the sensor element. This is advantageous to prevent the heater from being broken.
Furthermore, according to the features of the preferred embodiments of the present invention, it is preferable in the above-described first to fourth assembling methods that the metallic holder holds the heater at a single point.
As the heater abuts the bottom surface of the inside chamber, a force supporting the heater is generated from this abutting point. If the metallic holder holds the heater at a plurality of points, a force bending the heater will be generated. Therefore, to prevent the heater from being damaged, it is preferable that the number of supporting points of the heater is only one point.
Moreover, according to the features of the preferred embodiments of the present invention, it is preferable in the above-described first to fourth assembling methods that a lead wire be installed on the heater before the heater is inserted into the inside chamber of the sensor element. This is advantageous in that the installation of the lead wire onto the heater can be simplified.
Still further, according to the features of the preferred embodiments of the present invention, it is preferable in the above-described first to fourth assembling methods that the heater has a cavity at least on its tip end. Provision of such a cavity makes it possible to prevent the center of the tip end of the heater from colliding with the bottom of the inside chamber when the heater is brought into contact with the bottom surface of the inside chamber.
In general, a reaction force acting from the bottom of the inside chamber to the tip end of the heater is increased as the abutting point therebetween approaches the center of the bottom surface. Therefore, the sensor element can be prevented from being broken by providing the cavity on the tip end of the heater.
Even further, according to the features of the preferred embodiments of the present invention, it is preferable in the above-described first to fourth assembling methods that the heater have a groove axially extending on its outside surface. With this arrangement, when the heater is inserted in the sensor element, the referential gas can be sufficiently introduced into the innermost end of the inside chamber via the axially extending groove. Accordingly, the characteristics of the sensor element can be stabilized.
Moreover, according to the features of the preferred embodiments of the present invention, the heater can be formed into a polygonal shape. In this case, it is preferable that the corners of the polygonal heater abutting the metallic holder are chamfered or cut in a curved surface. It is recommendable that a ceramic spacer is interposed between the heater and the bottom surface of the inside chamber of the sensor element. And also, it is desirable that a guide plate is disposed beforehand at the open end of the sensor element to guide the metallic holder.